The invention relates to a tip material for a turbine blade or for a method of manufacturing or repairing a damaged tip of a turbine blade having a metallic coating.
Turbine components such as blades or vanes operate at high temperatures and under conditions of extreme environmental attack such as oxidation. The tip of turbine blades is normally the point of highest temperature on the part, and is especially subject to degradation by oxidation and/or wear. Conditions at the blade tip are often so extreme that ceramic thermal barrier coatings and oxidation resistant coatings experience limited lives and are ultimately consumed before the blade is removed from service for repair. It is therefore common for the tips of blades to require partial or complete replacement during the repair/reconditioning intervals.
As a result, several disclosures have been made regarding methods and materials to be used for the replacement procedure. There are generally two approaches to replacing the tip: removing by some method the damaged part of the tip until high quality material is exposed, then a progressive build-up using any welding overlay procedure or the brazing or welding of a previously manufactured coupon of material to the newly exposed surface of the tip. U.S. Pat. No. 5,794,338 discloses the practice of not removing coating adjacent to the tip face to be repaired, whereas standard practice was to remove such adjacent coating material up to several mm below the repair surface so that it does not interfere with the repair operations.
It is further known that the replacement material should be wear resistant or oxidation resistant as in U.S. Pat. No. 5,622,638, where a composition is given which is significantly different from the base composition of the underlying blade and has no specified relationship to the coating material used for the part.
In some disclosures, such as U.S. Pat. No. 4,832,252, materials for tip replacements are used that are compatible with or equivalent to the base material. However, these references do not disclose that there is any particular advantage to the materials being the same or different.
It is always problematic to find a Ni based alloy that will be compatible with a second Ni based alloy upon which it is deposited and left in contact at high temperatures for extended periods of time because of the inter-diffusion zone that forms between the two alloys. This problem is made worse when the two materials are mixed over a certain zone, as in welding. It is known that each precipitation strengthened nickel based alloy is carefully designed and balanced so as to precipitate a desired volume fraction of strengthening (but brittle) gamma prime as cuboidal precipitates separated by softer gamma phase. Each alloy is also carefully formulated to avoid the precipitation of harmful TCP (topologically close packed) phases, which seriously degrade high temperature properties. When two alloys are allowed to diffuse into each other across an interface, the careful balance of alloying elements is seriously disrupted in the region of inter-diffusion where the two alloys effectively mix. The result is often that continuous bands of brittle gamma prime form and there is relatively heavy precipitation of TCP phases (needles and/or plates), both serving as easy crack paths. Failure often occurs as cracks propagate not in the alloys, but in the inter-diffusion zone between the alloys.
This is particularly problematic when one alloy is rich in Al and Cr as in oxidation resistant materials, because increasing a base material composition in these elements promotes the strongest undesirable precipitates. It is particularly at the high temperatures experienced at the blade tip that gamma prime banding and TCP phases precipitation occurs the most rapidly and to the greater volume fraction, severely degrading high temperature properties. The aero-engine centered repair industry may not face this problem to a great extent due to the relatively short operating lives of aero-engine components between repair intervals. However, industrial and heavy-duty land based gas turbine components experience much longer continuous operating times between repair intervals and are more sensitive to such alloy-alloy interactions at the tip.
Accordingly, the invention provides a tip material that will minimize the harmful precipitation in the region of both the coating-tip and base material-tip interfaces.
In an embodiment of the invention, the material used for the tip is equivalent in composition to the coating material used for the alloy or a modified version of the coating composition in which the yttrium content (or other rare earth element) is controlled to smaller amounts, in the range of 5-1000 ppm, preferred range 15-200 ppm.
The advantages of the invention can be seen, inter alia, in the fact that the problems in the inter-diffusion zone between the material of the tip and the blade are minimized because the coating has already been developed to minimize problems in the inter-diffusion zone between the blade and the coating.
The precipitation of harmful TCP (topologically close packed) phases is avoided and therefore the danger of propagation of cracks in the inter-diffusion zone is reduced.
As no third materials are being used to replace the tip, there are no compatibility problems in the new tipxe2x80x94blade interface, or in the new tipxe2x80x94extra coating interface, as they are equivalent in composition.
Because of the intentional yttrium additions, which may either be the same as in the coating alloy (generally around 3000 PPM) or controlled to a smaller amount in the range of 5-1000 ppm, preferred range 15-200 ppm, it has been found unexpectedly that in said range of yttrium contents, weldability is improved to acceptable levels while oxidation resistance is maintained.
The advantage of the present invention over the prior art is that the replacement tip will not experience any welding incompatibilities with the original part coating to which it is welded, and minimal incompatibilities with the base material of the component itself, since the coating was designed to be in contact with the base material. Furthermore, the replacement tip will never need additional environmentally resistant coatings other than thermally insulating TBC coatings when desired. This saves considerable processing time and money during the repair of the component.
Moreover, a method of manufacturing or repairing a tip of a turbine blade having a metallic coating is further specified.
Even if the new tip material is welded or added directly to the coating, there are no problems in the mixing zone between the original coating and the new tip material as they are equivalent in composition. In addition to being perfectly compatible with the old coating, there are minimal incompatibility problems with welding or bonding to the blade material.